In the past, for example, the image display devices disclosed in Japanese Patent Application Kokai No. 2000-352689 and Japanese Patent Application Kokai No. 2001-264682 have been known as so-called see-through type head-mounted image display devices (head-mounted displays) which allow the user to view a display image superimposed on a view of the outside world while observing the conditions of this outside world. Furthermore, Japanese Patent Application Kokai No. 2001-264682 discloses not only a see-through type head-mounted image display device, but also an image display device that is not used as a see-through type device (i.e., that conducts only light from an image display element to the eyes of the user without superimposing other light from the outside world or the like on this light from the image display element) while having substantially the same construction as such a see-through type device; an example in which this image display device is contained in the flipper part of a portable telephone is also disclosed.
In these image display devices, a reduction in size and weight is achieved by using a reflective type holographic optical element. Such a reflective type holographic optical element is superior in terms of wavelength selectivity, and can selectively diffract and reflect only light in an extremely limited wavelength region. Accordingly, in cases where a see-through type image display device is constructed, loss of the amount of light that is transmitted from the outside world or the like by means of a reflective type holographic optical element can be conspicuously reduced.
Furthermore, in these image display devices, the exit pupil of the image combiner is formed so that this pupil substantially coincides with the pupil of the eye of the user in the use state, and a reflective type holographic optical element in these image display devices is manufactured so that the position of one light source (the reference light source) of the two light sources that are used to expose the reflective type holographic optical element during the manufacture of this element is caused to coincide substantially with the position of the exit pupil of the image combiner (paragraph No. 25 of Japanese Patent Application Kokai No. 2000-352689 and paragraph No. 37 of Japanese Patent Application Kokai No. 2001-264682). In these patent applications, the following effect is described: by using a reflective type holographic optical element which is manufactured with the position of the reference light source during manufacture disposed in the position of the pupil of the user, the exposure light during manufacture and the observation light during use substantially coincide, so that the diffraction efficiency of the reflective type holographic optical element during use can be improved to the maximum extent (paragraph No. 25 of Japanese Patent Application Kokai No. 2000-352689 and paragraph No. 37 of Japanese Patent Application Kokai No. 2001-264682).
Furthermore, in these image display devices, a liquid crystal display element is generally used as the image display element in order to achieve a reduction in size and weight, and an LED, which is a compact and inexpensive light source, is used as the light source that illuminates this image display element.
However, in these conventional image display devices, although a good display image can be viewed in cases where the center of the pupil of the eye of the user and the center of the exit pupil of the image combiner coincide, the display screen gradually becomes darker as the center of the pupil of the eye of the user moves from the center toward the periphery of the exit pupil of the image combiner, and the display image appears to be blurred, so that the image quality is not always sufficient. Furthermore, during actual use, it can frequently happen that the center of the pupil of the eye of the user deviates from the center of the exit pupil of the image combiner.